Microdisplacement Bug?
I have seen this a couple of times and it turned out to be one of the following:

the mesh is insufficiently subdivided (or not subdivided at all) before microdisplacement is executed

the mesh is overly subdivided before microdisplacment is executed

the max subdivisions setting is too low for the distance from camera to mesh (properties > render (set to experimental feature set) > geometry > max subdivisions)

the solution that worked for me in all cases was to find the sweet spot between too much and too little subdivision before I applied microdisplacement - in some situations (for reasons of available RAM) I was never able to get the visible mesh resolution to disappear entirely but it became a non-issue when motion blur was applied and/or when the render was viewed full frame (as opposed to very close in a pixel-peeping zoom-in)